Laser marking and laser engraving are well-known techniques which are frequently used in preparing identification cards and security documents. However in literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by the local heating of material causing carbonization. Gray shades can be obtained by varying the beam power. In laser engraving, the material is removed by ablation.
It is frequently mentioned in the literature that polycarbonate, PBT and ABS as polymers are laser-markable as such, i.e. in the absence of a so-called “laser additive”. However, laser additives are often added even in the case of these polymers in order to improve the laser markability further. A laser additive is a compound absorbing light at the wavelength of the laser used, usually at 1064 nm (Nd:YAG), and converting it to heat.
Carbon black can be used as a laser additive, however carbon black has a degree of colour which is sufficient to be visible prior to application of the laser beam and that can be unsightly or interfere with the distinctness of the mark after the laser beam has been applied. These disadvantages lead to a search for more efficient “colourless” laser additives. For example, U.S. Pat. No. 6,693,657 (ENGELHARD CORP) discloses a YAG laser marking additive based on a calcined powder of co-precipitated mixed oxides of tin and antimony which will produce a black mark contrasting with the surrounding area when exposed to YAG laser energy but prior thereto does not impart an appreciable colour to the surrounding area or cause a significant change in the performance of the material in which it has been added. Generally, the alternative laser additives are based on heavy metals making them less desirable from an ecological viewpoint.
Today, the most common plastic used in laser marking identification cards and security documents is a foil of extruded polycarbonate. However, polycarbonate foils have a number of disadvantages. The most important ones are their brittleness, leading to security cards getting broken when bended, and their lack of inertness towards organic solvents, opening possibilities to falsify a security card.
Polyethylene terephthalate (PET) exhibits a high solvent resistance, a high flexibility and is less expensive than polycarbonate, but exhibits no or very poor laser markability.
EP 866750 A (SCHREINER ETIKETTEN) discloses laser-markable films for labels based on a white PET film which bears a black coating. Laser irradiation ablates the black coating and uncovers the white background. This structure enables good high-contrast white-on-black inscriptions and drawings.
U.S. Pat. No. 7,541,088 (MITSUBISHI POLYESTER FILM) discloses a biaxially oriented, heat-set, at least two-layer coextruded film formed from polyethylene terephthalate (PET) or polyethylene 2,6-naphthalate (PEN) including a base layer and at least one outer layer. The base layer includes a white pigment and a laser absorber which has been coated with a carbonizing polymer. It is disclosed at col. 3, lines 64-66 that only the combination of the laser marking additive with a white pigment and with a specific coextruded layer structure leads to effective laser marking. The opaque coextruded layer structure prevents any security print, such as e.g. guilloches, present on a foil beneath to be visible through the laser markable layer structure.
WO 01/54917 (SIPIX IMAGING) discloses a heat sensitive recording material for thermal imaging comprising a transparent support sheet having a thermal slip layer disposed on one surface of the support and a heat sensitive color-producing layer on the opposite surface of the support wherein the color is formed from leuco dyes.
U.S. Pat. No. 5,407,893 (KONICA) discloses an ID card material comprising a thermal transfer image-receiving layer, and provided thereon, a substrate layer and a writing layer in this order, the substrate layer including a biaxially oriented polyester film layer having a thickness of 300 to 500 μm and a resin layer having a thickness of 30 to 500 μm selected from the group consisting of a polyolefin layer, a polyvinyl chloride type resin film layer and an ABS resin film layer.
EP1852269 (TECHNO POLYMER) discloses a laminate for laser marking which is useful for forming displays or indications, comprising a layer (A) and a layer (B) laminated on at least one side of layer (A), which layer (A) comprises a white or black coloring laser-marking thermoplastic resin, which layer (B) comprises a transparent thermoplastic resin and has a light transmittance of not less than 70% in the single layer, and the transparent thermoplastic resin in the layer (B) being subjected to anti-blocking treatment.
There is therefore a need for a transparent laser markable security film having a high solvent resistance and flexibility.